Hydraulic working machines such as excavators, track-type tractors, wheel loaders and the like use a plurality of hydraulic cylinders to provide actuating force for booms, sticks, buckets, and blades. Stroke or position sensors are commonly used to determine how far the rod has extended from the barrel of the hydraulic cylinder. The stroke information enables closed loop control of the actuation performed by the hydraulic cylinder. Closed loop control, in turn, enables automated digging operations. Accordingly, reliable stroke information is required to achieve reliable automated digging operations.
Hydraulic working machines operate in harsh environments. Temperature extremes, dust, dirt, high amplitude vibration, and shock are normal. The hydraulic cylinders, being on the business end of the machine, often experience the harshest conditions of the environment. The harsh environment reduces the reliability and durability of components that are intended to measure the stroke of a hydraulic cylinder. Optical methods can be obscured by dust. Waveguides and detectors can succumb to vibration. Hall effect sensors often use contact methods (odometry) to detect stroke and are susceptible to dust and hydraulic oil.
For example, U.S. Pat. No. 6,710,327 to Arshad et al., issued Mar. 23, 2004, entitled “Multi-fiber multi-cylinder position method and apparatus using time-of-flight technique,” discloses an apparatus for measuring the time of flight of a light pulse for determining the stroke of a cylinder. The operation of the system, however, can be compromised by dirt or other contamination accumulating on the photodiode that receives the light signal.
A second example, U.S. Pat. No. 7,757,547 to Kageyama et al., issued Jul. 20, 2010, entitled “Cylinder stroke position measurement device,” discloses an apparatus for determining the stroke of a cylinder using a sensor wheel and a Hall effect sensor. The Kageyama apparatus however is susceptible to dirt that could cause binding or slipping of the sensor wheel. The Kageyama apparatus is further susceptible to hydraulic oil that could cause slipping of the sensor wheel which could cause an inaccuracy in the cylinder stroke measurement.
A system and method for measuring the stroke of a cylinder that is less susceptible to dirt and contamination is required.